1. Field of Invention
The present invention relates to an electronic timepiece and to a time adjustment method for an electronic timepiece that receives signals transmitted from a positioning information satellites such as a GPS satellite and adjusts the time.
2. Description of Related Art
The Global Positioning System (GPS), which can be used to determine one's location, uses GPS satellites that orbit the Earth on known orbits with each GPS satellite having an on-board atomic clock. As a result, GPS satellites also transmit extremely accurate time information (referred to herein as GPS time or satellite time information).
Electronic timepieces that use time information from a GPS satellite to adjust the time kept by the timepiece are also known from the literature.
In order to acquire the necessary time information, an electronic timepiece that uses time information from a GPS satellite receives the week number WN (information identifying the week to which the current GPS time belongs), the time of the week TOW (Time of Week), and time information, also called the Z count, identifying the current day and time in the week identified by the week number in seconds counted from the beginning of each week. The accurate current time can then be calculated from the received week number and time of week information.
The week number WN is a number that started at 0:00:00 on 6 Jan. 1980 and increments 1 every week. The week number is a 10-bit digital value that therefore resets to 0 every 1024 weeks (approximately 19.7 years), a phenomenon known as week number rollover.
The current date (year, month, day) can therefore not be accurately determined using the week number WN after 1024 weeks from 6 Jan. 1980 0:00:00 h.
To solve this problem, Japanese Unexamined Patent Appl. Pub. JP-A-2001-228271, Japan Patent No. 3614713, and Japanese Unexamined Patent Appl. Pub. JP-A-2002-90441 teach timekeeping devices that acquire a reference date or other information from an external source, and calculate the accurate date based on this reference date and the week number WN and time of week TOW received from a GPS satellite.
The timekeeping device taught in JP-A-2001-228271 reads the reference date from a removable medium storing the reference date information, acquires the time by accessing the Internet, or acquires the reference date from the reference date input from a screen input device. It then converts the GPS time to a year-month-day-hour-minute-second format assuming that the GPS time is within 1024 weeks of the acquired reference date, and calculates the UTC time. As a result, the timekeeping device taught in JP-A-2001-228271 can calculate the year, month, day based on the input reference date if a new reference date is input once every 10-plus years, and can semipermanently calculate the correct date (year, month, day).
The GPS receiver taught in Japan Patent No. 3614713 calculates the WN cycle number based on a user setting or the week number WN stored on a map data storage medium. The cycle number is the number of times the 10-bit week number WN changes from 0 to 1023. If this cycle number is known, the correct date can be acquired from the acquired WN information.
The GPS receiver taught in Japanese Unexamined Patent Appl. Pub. JP-A-2002-90441 receives a standard time signal, acquires time information, recognizes the correct Gregorian calendar year using the last two digits of the Gregorian year in the time information based on the standard time signal, and can correct the time information using the recognized correct Western year number. By using the last two digits of the Gregorian year received from a standard time signal, the correct Gregorian date can be recognized for at least 100 years from the start of when the GPS system started went into service.
With the method of acquiring a reference date from a removable medium as taught in JP-A-2001-228271, however, water resistance is impaired by the need to provide a connector for inserting the removable medium, device size is increased by the size of the medium, and using this method in a wristwatch is thus difficult. In addition, when time information is acquired through Internet access, use is limited to places where there is Internet access, and where the method can be used is therefore restricted. Yet further, when the year, month, and day of the reference date is input from a screen input device, there is much information to input and ease of use is not good. Usability is particularly poor with an analog wristwatch because the hands or other device must be manipulated to input the year, month, and day.
Yet further, the method taught in JP-A-2001-228271 cannot determine the correct year, month, day if the reference date is not updated at least once within the 1024 weeks.
When the WN cycle number is set by the user as described in Japan Patent No. 3614713, the user must have knowledge of the GPS system in order to determine the current cycle number, and usability is poor. In addition, when the WN cycle number is acquired from a map information storage medium, a mechanism for reading information from the storage medium must be provided, the system configuration thus becomes complicated, and use in a small timepiece such as a wristwatch is difficult.
When time information acquired from a standard time signal is used as taught in JP-A-2002-90441, use is limited to places where a standard time signal can be acquired. Yet further, a standard time signal reception unit for receiving the standard time signals must be provided in addition to a GPS receiver, thus complicating the configuration, making reducing device size difficult, and making use in a small timepiece such as a wristwatch particularly difficult.